I'm much more sympathetic to arguments about what's good for people (consumers) than I am to what's good for competitors.

I think people would be made worse off if Amazon's retail operations were to be made worse (or shut down in the limit case).

For Amazons customers, yes - but competing offerings might be better off - by creating a gap in the market that might be filled by more motivated businesses.

"I think people would be made worse off if Amazon's retail operations were to be made worse (or shut down in the limit case).

"

Maybe short term but the in the long term I think everybody would be better off with more smaller competitors instead of a few dominant ones. Same for Apple. I don't think their size is good for the economy or technology in the long run.

Isn't that arguing in favor of unfair competition (specifically undercutting), in a roundabout way?

One person's undercutting is another person's competition.

There's nothing wrong or anti-competitive about undercutting another retailer. At some point, you can get into anti-competitive predatory pricing or "dumping", but I don't think anyone is credibly arguing that Amazon is dumping product below their cost.